When researchers recently reviewed more than a decade’s worth of findings, they concluded that so-called mega-doses of vitamin C don’t prevent colds for most people. Some studies suggest vitamin C may help you get over the symptoms a little faster. But vitamin C doesn’t appear to lessen its intensity.

Studies show that a variety of nutrients may help lower the risk of age-related macular degeneration, the leading cause of vision loss in people over 65. Eating a diet loaded with fruits and vegetables should give you all you need. But if you are at increased risk of age-related macular degeneration, your doctor may recommend a supplement as well as lifestyle changes such as quitting smoking.

Vitamins are compounds that are vital to our health. They were first thought to be amino acids, the building blocks of protein. Today, we know that 13 vitamins are essential to human health. You can usually get all you need by eating a well-balanced diet. But some people, especially those on restrictive diets, may benefit from vitamin supplements. How much do you need? The government has established recommended daily allowances for most vitamins and minerals, as well as upper limits that are safe.

Getting enough B12 is definitely important. A lack of B12 can cause anemia, but the symptoms of B12 deficiency are not typically related to feeling weak and tired. Symptoms do include memory loss, disorientation, and tingling of your arms and legs. But there’s little evidence that taking B12 makes you a better athlete or gives you more energy. An adequate diet that includes meat, fish, or dairy products will give you all the B12 you need.

Most people know that calcium and vitamin D are essential for healthy bones. New findings show that many other nutrients also are critical for bone health, including magnesium, potassium, and vitamin K. It’s best to get these nutrients from food. But if you’re at increased risk of brittle bones and osteoporosis, your doctor may recommend taking a calcium and vitamin D supplement.

Vitamins are essential for growth, digestion, and nerve function. Minerals support cell structure and help build bones and regulate body processes. Vitamins are known by letters of the alphabet: A, B, C, D, E, and K. Examples of minerals include calcium, which builds bone and helps blood clotting, and iron, which helps your blood carry oxygen.

Some water soluble vitamins, including C and B vitamins, turn your urine a bright orange or even yellow-green if you take more than you need. That’s your kidneys getting rid of the excess levels of these vitamins. But most vitamins and minerals do not show up in the urine, and it can be dangerous to get too much of some nutrients.

Pregnant and breastfeeding women often need a bigger variety of nutrients. Other people also may need vitamin or mineral supplements for a variety of reasons. Those on special diets, such as vegans, may not get all the vitamins they need from food. Others whose bodies may have trouble absorbing all the nutrients they need can include people:

• With certain medical conditions, such as inflammatory bowel disease
• Who have had some types of weight loss surgery
• Who take some medications for seizures and heartburn
• Who frequently use aspirin

Talk to your doctor if you think you may need a supplement.

Supplements can be a good way to ensure you’re getting all the nutrients you need, however, many vitamins and minerals can be toxic if you take too much. Excess vitamin A can cause nausea, vomiting, and liver damage, for example. Too much vitamin D can cause weakness, heart rhythm problems, and confusion. Because the body stores fat-soluble vitamins [A, D, E, and K] and iron, consuming too much can cause levels to build up in organs and tissues, causing liver or kidney damage.

Although citrus fruits are the best known source of vitamin C, many fruits and vegetables also contain abundant levels of C. Most adults need between 75 and 90 mg a day. Half a cup of sweet red pepper or a 6-ounce glass of orange juice provides that much and more.

This crucial B vitamin is found mostly in foods from animal sources, including meat, eggs, and dairy foods. If you don't eat these foods, you may not be getting enough B12. Talk to your doctor about whether you should take a supplement.

Researchers continue to study whether some antioxidants may prevent the kind of genetic damage that can turn cells cancerous. But there is no evidence that taking antioxidant supplements will lower your cancer risk. If you’re being treated for cancer, talk to your doctor before taking any dietary supplements. Some research suggests that antioxidant supplements may interfere with some cancer-fighting medicines.

Women who are menstruating usually need more iron because they lose a little with each menstrual cycle. The recommended daily allowance for premenopausal women is 18 mg a day. By contrast, men usually need only 8 mg of iron a day. Pregnant women should get 27 mg a day.

Experts are deeply divided on multivitamins. Several recent studies have shown no benefits of taking multivitamins in a healthy person. Even people who have taken a daily multivitamin for years don't appear to be healthier or to live longer. But other experts believe that taking a multivitamin can help fill in for what's missing in many people’s diets. Experts do agree that the best source for vitamins and minerals is food, not pills.

These are all different forms of B vitamins. B vitamins do many things in your body, including helping change food into energy. Whole grains, beans, nuts, meat, eggs, and fish are good sources of many different forms of B vitamins. Vitamin B12, on the other hand, is found almost exclusively in foods from animal sources.

People with dark skin may be more likely to have low vitamin D levels. Older people may also be deficient, because your skin gets less efficient at converting sunlight to vitamin D as you age. If you stay out of the sun because of sensitive skin or the risk of skin cancer, you may also need extra D. Talk to your doctor. A simple blood test can show whether you are deficient in vitamin D.

Folic acid helps prevent neural tube defects, which involve the brain and spinal cord and can cause devastating birth defects. Because folic acid is so important during pregnancy, some doctors recommend that pregnant women and women who are thinking of becoming pregnant take a supplement to ensure that they get enough.

This mineral helps keep your heart beating regularly, and one study found that taking potassium reduced systolic blood pressure (the top number). Reducing blood pressure lowers the risk of stroke, heart disease, heart failure, and kidney disease.

The recommended intake of potassium for teens and adults is 4,700 mg a day. Many foods contain potassium, so a deficiency is rarely caused by an inadequate diet. Potassium-rich foods include many fruits and vegetables, milk, meat, and whole grains.